Since the late 1960's it has been customary to utilize a plastic cement to fixedly secure the substitute hip components employed in total hip joint replacement surgery. It has been found, however, that this method of fixing the components into the patient's surrounding bone has serious drawbacks because the cement often becomes brittle and cracks after being in place a few years. As a result the substitute components loosen and migrate from their proper positions thereby causing both pain and deformity and necessitating further corrective surgery.
To avoid these problems it is now preferred to utilize other methods of attaching the substitute hip components to the patient. A frequently used method for fixing the acetabular component into the hip bone socket cavity, the acetabulum, is to form screw threads on the outer surface of the acetabular component so that it can be screwed directly into the previously prepared acetabulum. The exterior shape of the substitute acetabular components currently available vary considerably, but generally are hemispherical, truncated cones or cylinders, with screw threads of various size, shape and pitch formed on their outer surfaces. Preparation of the acetabulum is accomplished by chiseling, reaming or scraping the interior of the roughly hemispherical hip socket cavity to remove cartilage and some or all of the bony wall until the acetabulum is modified to substantially conform to the size and shape of the acetabular component that will be fitted into it. While some acetabular components have self tapping threads on their exterior surfaces to permit the component to be screwed directly into the prepared acetabulum, other components require that the bony wall be threaded with a tapping device before the acetabular component can be inserted into the hip socket cavity.
As presently done, this threading operation is carried out using a hand tap or plug having the same size and shape as the prepared acetabulum with cutting threads affixed to the plug's external surface. The use of a conical hand tap requires that the threads be cut simultaneously around the wall of the acetabulum, a very difficult task that requires a great deal of physical force. Indeed, despite great force, it is difficult to cut full depth threads into the relatively hard cortical layer of bone which lines most of the acetabulum. The force of turning the conical tap into the bony wall can cause expansive strains on the walls of the acetabulum causing it to fracture and prevent mechanical fixation of the component. In addition, if the threads are not of full depth and of the proper size and shape, the threaded acetabular component will not seat in the socket properly or deeply enough. Accordingly, it is customary in preparing the acetabulum to cut away most of this hard cortical outer layer of bone in the chiseling and reaming operation to expose the much softer and spongy cancellous bone beneath it. Although it is much easier to create screw threads in this softer cancellous bone it does not provide as strong a supporting medium for anchoring the acetabular component as the hard cortical layer and the risk that the component will loosen in time is much greater.
The objective of the present invention is to provide a device for cutting threads in the acetabulum that does not have the drawbacks and disadvantages discussed above. Another object of the invention is to provide a tool that will enable threads to be easily cut in the hard cortical layer of the acetabular cavity.
Still another object of the invention is to provide a tool whose cutting head can be readily modified to match the contours of a bony hip socket that has been reamed and shaped to conform to the size and shape of a substitute acetabular component.